Which one of the following is not true about the image formed by a plane mirror?
- (a)It is of the same size as the subject.
- (b)It is laterally inverted.
- (c)It is real image.
- (d)It is formed as far behind the mirror as the object is in front.
Correct — C, It is real image. NCERT's summary of the plane mirror runs the other way: the image is always virtual and erect, its size equals that of the object, it lies as far behind the mirror as the object is in front, and it is laterally inverted. Three of those four appear in this question as options (a), (b) and (d), all true. What is left is the claim that the image is real, and it is false. Reflected rays from a plane mirror diverge after reflection and never actually meet; they only appear to come from a point behind the mirror, where no light reaches. An image at a place light cannot reach cannot be caught on a screen, and that is what virtual means.
- (a)It is of the same size as the subject. — True — the magnification of a plane mirror is exactly one, so the image is neither enlarged nor diminished. The paper writes 'subject' where the rest of the question and the textbook say 'object'; the meaning is unaffected.
- (b)It is laterally inverted. — True, and it is the reason writing held up to a mirror reads backwards and an ambulance carries its name reversed on the bonnet.
- (d)It is formed as far behind the mirror as the object is in front. — True. Put an object 30 cm in front and the image sits 30 cm behind, giving an object-to-image separation of 60 cm.
A real image is formed where reflected or refracted rays actually converge, so it can be thrown on a screen and is inverted. A virtual image is formed where the rays only appear to converge when produced backwards; it cannot be caught on a screen and stays erect. A plane mirror always gives the second kind — virtual, erect, the same size as the object, laterally inverted, and as far behind the mirror as the object is in front.
Not-true items are easiest when you can recite the positive list and cross off. The list for the plane mirror has exactly four members and three of them are printed here, so the fourth option is the answer by elimination even before you think about ray diagrams. Worth noticing in passing is that the paper writes 'the same size as the subject' in option (a) while option (d) says 'object' — the booklet's own wording, and one of the small slips that survive into a printed paper. It does not touch the reasoning. If you would rather argue from first principles than from a list, a plane mirror has an infinite focal length, so the mirror formula gives an image distance equal and opposite to the object distance, and a magnification of plus one — erect, same size, virtual.
- The image in a plane mirror is virtual and erect, of the same size as the object, and laterally inverted.
- It lies as far behind the mirror as the object stands in front of it.
- A real image can be caught on a screen; a virtual image cannot, because no light actually reaches it.
- A plane mirror can be treated as a spherical mirror of infinite radius of curvature, so its focal length is infinite and its magnification is plus one.
- A mirror of half a person's height, correctly placed, is enough to show the full-length image, whatever the distance.
- Calling the image real because you can plainly see it; visibility is not the test, catching it on a screen is.
- Believing a bigger mirror gives a bigger image; the magnification of a plane mirror is always one.
- Assuming lateral inversion means up and down are swapped as well — only left and right appear exchanged.
As a which-is-not-true item on the plane-mirror image, or as a direct 'the image formed by a plane mirror is ___' with a three-adjective answer.
The image of an object formed by a plane mirror is
- (a) erect, real and larger.
- (b) erect, virtual and same size.
- (c) inverted, virtual and same size.
- (d) inverted, real and smaller.
Answer(b) erect, virtual and same size.
The positive form of this very item. There the task is to pick the three adjectives that fit the plane-mirror image — erect, virtual and same size — while here the task is to spot the fourth adjective that does not.
- practice — not a real PYQ
An object is placed 25 cm in front of a plane mirror. The distance between the object and its image is
- (a)12.5 cm
- (b)25 cm
- (c)50 cm
- (d)75 cm
Answer(c) 50 cm — the image lies 25 cm behind the mirror, so object and image are 25 plus 25 centimetres apart.
- practice — not a real PYQ
The focal length of a plane mirror is
- (a)zero
- (b)equal to its length
- (c)infinite
- (d)equal to the object distance
Answer(c) infinite — a plane surface is a spherical mirror whose radius of curvature has grown without limit, and focal length is half that radius.